This invention relates to power control circuits suitable for varying the power supplied to an appropriate load and, more particularly to a solid state triggering circuit for a microwave oven power supply for varying the intensity of the microwave power.
Over the past several years, the microwave oven has become a familiar appliance. Many different types of ovens having varying functions associated therewith have been manufactured. A typical microwave oven and power supply circuit is illustrated in U.S. Pat. No. 3,780,252 to Crapuchettes et al. As brought out in this patent, all microwave ovens employ as a source of microwave energy, a microwave tube or "magnetron". The magnetron requires a high voltage power supply to generate microwave energy that includes a high voltage transformer to step up the line voltage carried to the homes of the consumer. As described in the subject patent, one such power supply circuit utilizes a high leakage reactance transformer and a voltage doubler circuit including a doubling capacitor. Briefly, by charging the doubling capacitor during a portion of the applied line voltage cycle, sufficient potential is developed during another portion of the applied line voltage cycle which causes the magnetron to be rendered conductive in timed relationship to the applied line voltage such that microwave energy is generated to cook food articles.
The subject patent is directed towards providing a phase control circuit for determining the proper phase at which current is supplied to the transformer whenever the oven is initially turned on. As disclosed, the current is synchronized with the applied line voltage in an attempt to reduce in-rush current transients. Therefore, during each desired cooking cycle (as set by a mechanical timer) the magnetron is rendered conductive in relation to to the applied line voltage until such time that the timer turns the oven off, which could be during any portion of the alternating cycle of the applied line voltage.
The Crapuchettes circuit has the disadvantage that during the complete time interval, as set by the timer, the magnetron conducts on alternating half cycles of the applied line voltage. Therefore, the net average power produced for heating food articles can not be varied in any particular cooking cycle period. However, it is desirable with many foods to set the cooking cycle for a definite time period and then adjust the average power produced by the microwave source so that more uniform cooking is obtained. Further, for example, to defrost foods it is necessary to have the microwave oven on for a relatively long period with the effective intensity of heat generated being nominal so that the food is not cooked as it is being thawed.
According to another prior art system, U.S. Pat. No. 3,862,390 to Noda, there is disclosed a circuit for varying the intensity of the microwave energy supplied by the energy source (a magnetron, for example). As brought out, the time period of the high frequency output energy from the magnetron can be varied to change the intensity of heating waves which are used to cook food articles. By varying the time period, more or less energy is supplied selectively so that energy of the proper intensity is derived in cooking different types of foods. For example, in the preparation of scrambled eggs, more uniform cooking is derived by reducing the time period at which microwave energy is produced during the cooking cycle. Further, it is not desired to defrost frozen foods at a high intensity as the frozen food article will not only be defrosted but also would be partially cooked if the microwave energy is not reduced during the time that the microwave oven is turned on.
The above described circuit as well as other prior art microwave ovens which include additional timer circuits for varying the effective intensity of the microwave energy have suffered in that audible clicks or "thumps" are emanated during the cooking cycle. It has been found that in addition to synchronously causing current to flow for deriving a voltage to produce the microwave energy that the current should be synchronously shut off with the applied line voltage. As will be discussed hereinafter, current surges into the high voltage transformer of the microwave frequency power supply can be eliminated thereby eliminating the above mentioned audible noise. Further, by reducing this current surge, semiconductor devices having smaller power ratings can be used thereby reducing component costs and hence, production costs which are directly passed to the consumer.
Thus, a need exists for a power control circuit for solving the above problems of transients and current surges. By elimination of such transients, a significant cost reduction in the manufacturing of microwave ovens is made possible.